Austenitic steel combining strength and resistance to intergranular corrosion

ABSTRACT

Austenitic stainless steel characterized by a combination of good hot-rolling and good cold-rolling properties, good strength and ductility, and good welding properties, with resistance to intergranular corrosion in the as-welded condition. The steel contains about 17.5 to 22.5 percent chromium, about 5 to 9.5 percent nickel, at least 2.5 percent but less than 7 percent manganese, with a nitrogen content of about 0.2 to 0.4 percent, carbon not exceeding 0.040 percent, and remainder substantially iron. Where desired, there may be employed columbium in amounts up to about 1 percent and/or molybdenum up to about 3 percent. The steel is made available in the form of hot and cold flatrolled products, as well as wire. It is suited to a wide variety of applications in the arts.

AUSTENITIC STEEL COMBINING STRENGTH AND RESISTANCE TO INTERGRANULAR CORROSION Elbert E. Denhard, Jr., Towson; Robert R. Gaugh, Lutherville, both of Md.

Armco Steel Corporation, Middletown, Ohio Filed: May 2, 1969 Appl.No.: 821,460

lnventors:

Assignee:

References Cited UNITED STATES PATENTS Feb. 29, 1972 3,235,378 2/1966 Jennings ..75/128 N 3,284,250 11/1966 Yeo ....75/l28 N 3,311,511 3/1967 G0ller.. ....75/l28 N 3,401,036 9/1968 Dulis ..75/l28 N Primary Examiner-Hyland Bizot Attorney-.lohn Howard Joynt [57] ABSTRACT Austenitic stainless steel characterized by a combination of good hot-rolling and good cold-rolling properties, good strength and ductility, and good welding properties, with resistance to intergranular corrosion in the as-welded condition. The steel contains about 175 to 22.5 percent chromium, about 5 to 9.5 percent nickel, at least 2.5 percent but less than 7 percent manganese, with a nitrogen content of about 0.2 to 0.4 percent, carbon not exceeding 0.040 percent, and remainder substantially iron. Where desired, there may be employed columbium in amounts up to about 1 percent and/or molybdenum up to about 3 percent. The steel is made available in the form of hot and cold flat-rolled products, as well as wire. It is suited to a wide variety of applications in the arts.

4 Claims, No Drawings AUSTENITIC STEEL COMBINING STRENGTH AND RESISTANCE TO INTERGRANULAR CORROSION As a matter of introduction, our invention generally relates to the stainless steels, particularly those characterized as being austenitic.

One of the objects of our invention is the provision of a stainless steel which readily lends itself to working in the hotmill, and in the cold-mill as well, as for example, in the production of flat-rolled products such as plate, sheet and strip, as well as bars, rods and wire.

Another object is the provision of such steel which readily lends itself to a variety of forming operations, notably cold-upsetting, bending, drawing, cutting, threading, and the like, as well as fabricating operations, such as welding and brazing, as in the production of various articles of ultimate use.

A further object is the provision of a fully austenitic stainless steel and products fashioned thereof which enjoy a combination of strength, along with excellent corrosion-resistance to a wide variety of corrosive media even in the as-welded or as-brazed condition, that is, in the welded or brazed condition without necessity for subsequent annealing or other heat treatment.

Other objects of our invention will become apparent during the course of the description which follows, others being particularly pointed to.

Our invention, then, will be seen to reside in the combination of elements, in the composition of ingredients, and in the relation between the same, as well as in the products and articles fashioned thereof, all as more fully described herein and as more particularly set forth in the claims at the end of this specification.

CROSS-REFERENCE TO RELATED APPLICATION The present application is a companion of the copending application of Denhard, Perry and Gaugh, Ser. No. 725,516, filed Apr. 30, 1968 and entitled Austenitic Stainless Steel.

BACKGROUND OF THE INVENTION As an aid to a better understanding of certain features of our invention, it may be well to note at this point that the austenitic grades of stainless steel are well recognized in the arts and are suited to a wide variety of applications where resistance to corrosion is required. In all of these steels chromium, of course, is an essential ingredient, this usually amounting to some to 30percent. Nickel, too, is essentially required, this ranging from some 4 or 5percent on up to some to percent. Although manganese is not essentially necessary, this ingredient may be present in amounts up to some 15 percent, this in some instances for use as a partial substitute for nickel. The residuals silicon, phosphorous, sulphur and nitrogen commonly are in low amount, although they may be employed in substantial amount for particular purposes. In some grades there additionally may be present for special purposes one or more of cobalt, molybdenum, tungsten, vanadium, copper, aluminum, columbium and titanium.

Several of the more popular grades of the austenitic stainless steels are the Type 302 (17 to 19% chromium, 8 to 10% nickel, and remainder iron), the Type 304 (18 to 20% chromium, 8 to 12percent nickel, and remainder iron), the Type 309(22 to 24 percent chromium, 12 to 15 percent nickel, 2 percent manganese, and remainder iron), the Type 310 (24 to 26% chromium, 19 to 22% nickel, 2% manganese, and remainder iron), the Type 316 (16 to 18% chromium, 10 to 14% nickel, 2 to 3% molybdenum, and remainder iron), the Type 321 (17 to 19% chromium, 8 to 11 percent nickel, 0.4% titanium, and remainder iron), and the Type 347 (17 to 19% chromium, 9 to 12% nickel, 0.8% columbium, and remainder iron).

More recently, there has been developed the Armco-type 21-6-(19 to 21% chromium, 6 to 7% nickel, 8 to 10% manganese, 0.25 to 0.35% nitrogen, 0.04% max. carbon, and remainder iron). This steel forms the subject of the Jennings U.S. Pat. No. 3,235,378 of Feb. 15, 1966. And there is the 206- (19.0 to 21.0% chromium, 5.0 to 7.0% nickel, 7.0 to 9.0% manganese, 0.25 to 0.35% nitrogen, 0.10% max. carbon, and remainder iron) which is described in the Renshaw, et al. article titled The Corrosion Properties of Chromium-Nickel- Manganese Austenitic Stainless Steels" appearing in the Proceedings American Society for Testing Materials, Vol. 56, 1956, pages 866 et seq.

Although all of the austenitic stainless steels enjoy excellent resistance to corrosion in a variety of applications, and generally are found to work well in both the hot-mill and the cold-mill, a number are lacking in strength. And many are not suited to welding applications, that is, applications where the steel in the form of various converted and shaped formed products require welding. And especially they are not suited to applications where the products and articles must be put to use without benefit of annealing or other heat treatment. In short, none of the prior steels enjoys a combination of strength, general corrosion-resistance and resistance to intergranular attack.

Accordingly, it is an object of our invention to provide an austenitic stainless steel in which there is had a combination of good hot-working properties, good cold-working properties and good forming properties, as well as good welding and good brazing properties, together with strength, ductility and toughness and resistance to a variety of salts and acids even in the as-welded condition, that is, without necessity for heat treatment subsequent to welding.

SUMMARY OF THE INVENTION Turning now to the practice of our invention, we provide an alloy steel essentially consisting of the ingredients chromium, nickel, manganese and nitrogen, with remainder substantially all iron. And while there is some latitude in the amounts of these several ingredients which are employed in our steel, we feel that the limits of each are, indeed, critical. For where the lower limitsare departed from, or, indeed, departure is made from the upper limits, one or more of the desired properties immediately suffer, all as more particularly dealt with below.

In our steel the chromium is employed in the amount of about 17.5% to about 21.5% or even to about 22 or 22.5% more particularly about 18 or 19% to about 22.5%, the nickel in the amount of about 5, 6 or 7% up to about 9.1% more especially about 6 or 6.5 or even 7% on up to about 7.5 or 8%, the manganese in the amount of about 2.5 or even 3 or 4% up to about 6 or 6.5 or even 6.7 or 6.9% but in any event less than 7%, with the nitrogen in the amount of about 0.2 to 0.3 or 0.4% more particularly about 0.25% to about 0.35%, and with remainder substantially all iron.

The carbon content of our steel is maintained in critically low amount, namely, not exceeding 0.035%, certainly not exceeding 0.040%, and for best results not exceeding 0.030 or 0.025 or even 0.020%. Molybdenum up to 3% and/or columbium up to 0.6 or 1% may be employed in our steel. Where columbium is present up to about 0.6 or even to about 1%, the carbon content may amount to as much as 0.040% as a maximum. Silicon, phosphorous and sulphur also are low.

Where the chromium content of the steel is less than about 18%, and certainly where it is less than about 17.5%, there is a loss of corrosion -resistance. And where it exceeds about 22.5% the steel is inclined to the formation of delta-ferrite, with consequent loss of hot-working properties; best results are had when the chromium content does not exceed about 22%, or better yet, when it does not exceed about 21 or 21.5% for an assured freedom from delta-ferrite without necessity for rebalancing the steel with additional nickel, this at significant cost.

The nickel content of the steel of our invention must be at least 5% in order to assure an austenitic structure, it preferably amounting to about 6% or even 6.5% for that purpose. Nickel exceeding about 9.5%, however, appears to result in a loss of resistance to intergranular corrosive attack.

This we attribute to a decrease in the solubility for carbon and carbides which may form. Offsetting any adverse effect on the resistance to intergranular corrosion, however, there is further increased stability of the austenite, with suppression of any tendency toward ferrite formation, all with a direct improvement in impact resistance.

Now the manganese content of the steel of our invention is, indeed, critical, particularly as to its upper permissable limit. At least about 2.5 or 3% manganese, and more preferably about 4 or 5%, is necessary to the austenite stability of the metal. Manganese is an amount less than about 3%, and certainly where it is in an amount less than about 2.5%, is inclined to result in a gassy steel. This we attribute to the circumstance that there then is insufficient manganese to support the high quantity of nitrogen required. Where the manganese content exceeds about 4%, however, and certainly where it amounts to 6 or 6.5%, a greater amount of nickel is required to maintain the austenitic structure. Also we find that the greater amount of nickel, say some 7.5 to 9.5%, is required to retain a desired resistance to intergranular attack, as appears more fully hereinafter. With a manganese content of 7% or more, the resistance to intergranular corrosion suffers, particularly where the steel is in the sensitized condition, that is, in the condition where part of the steel is brought to a temperature of some l,250 F. and then cooled. And, moreover, we find that an increase in manganese above the maximum permissible limit, there is a tendency toward the development of ferrite, even where the nickel content is high,.this with sacrifice to the austenitic structure, as evidenced by a loss of impact strength at subzero temperatures.

A nitrogen content of at least 0.2%, or better still, at least 0.25%, is necessary to lend a desired yield strength to our steel. But nitrogen in excess of about 0.35%, and certainly in excess of about 0.4%,results in a loss of resistance to intergranular attack in both the annealed condition and the sensitized condition as well.

In our steel the ingredient carbon, which is commonly present in all stainless steels, is found to be most critical. For with any increase in carbon above 0.035%, and certainly above 0.040%, the resistance of the steel to intergranular corrosive attack is virtually lost, the steel in many instances actually dissolving in the corrosive media. Best results in matters of resistance to intergranular attack and yet good retained strength and toughness are had where the carbon content does not exceed 0.030%, particularly good results being had where the carbon content does not exceed 0.020% as a maximum. Carbon contents of as much as 0.035% and 0.040% are acceptable only where columbium is employed, this in amounts of some 10 or 12 times the carbon content, and then only up to about 1%.

Columbium improves the resistance of the steel to intergranular attack and preferably is employed. Moreover, columbium seems to refine the grain structure and to significantly increase the strength of the metal. But columbium is a strong ferrite former, and with its presence, even in the amounts noted, the steel must be rebalanced by maintaining the nickel and nitrogen contents at sufficient levels to prevent the formation of ferrite.

The molybdenum addition generally improves the resistance to most corrosive media. But the sum of the molybdenum and chromium contents of our steel should not exceed about 22.5 percent, for otherwise the structural balance of the metal is adversely affected.

While the silicon content of our steel is not so critical as the carbon content, we find that best results are had where silicon is employed only in amounts up to about 0.5%; where resistance to the combustion products of leaded fuels is to be encountered, the silicon content should not exceed 0.25%. The low-silicon level also is desired in order to favor the austenitic balance of the steel.

DESCRIPTION OF THE PREFERRED EMBODIMENTS ln broad composition the steel of our invention, as generally noted above, essentially consists of chromium in the amount of about 17.5% to about 22 or 22.5%,nickel in the amount of some 5% up to about 9.5%, manganese at least about 2.5 up to about 6.9% but in any event less than 7%, nitrogen in the amount of 0.2% to about 0.4%, with remainder substantially all iron. The ingredients carbon, silicon, phosphorous and sulphur of course are present, the carbon content for best results being in an amount not exceeding 0.030%, and better yet not exceeding 0.020%, but in no event in excess of 0.040% even with columbium present. Silicon is present in amounts up to about 0.5% and preferably not over 0.25%. The phosphorus and sulphur, contents are low, the phosphorus generally not exceeding 0.03% and the sulphur generally not exceeding 0.02%. Where desired, molybdenum may be employed, this in amounts up to about 3%, particularly about l.5% to about 2.5%, and columbium, as noted, may be employed in amounts up to 1%.

While the composition of our steel is broadly defined as indicated, there are a number of specific steels in which a best combination of properties is bad. One such steel essentially consists of about 20% to about 22.5% chromium, about 5% to about 7.5% nickel, about 2.5% to about 5% manganese, about 0.25% to about 0.4% nitrogen, carbon not exceeding 0.030%, and remainder substantially all iron. Another essentially consists of about 20% to about 22% chromium, about 7.5% to about 9.5% nickel, about 2.5% to about 4% manganese, silicon up to about 0.5%, carbon not exceeding 0.030%, nitrogen about 0.25 to about 0.35%, and remainder substantially all iron. These steels enjoy a good combination of general corrosion-resistance, resistance to intergranular corrosion and good strength.

Another steel, this enjoying a combination of strength, toughness and resistance to intergranular corrosive attack, essentially consists of about 17.5% to about 22.5% chromium more especially about 19 or 20% to about 22.5% chromium, about 6% to about 9.5 nickel, about 4% to about 6.5% but in any event less than 7% manganese, about 0.25% to about 0.4% nitrogen, carbon not exceeding 0.030%, with or without columbium up to about 0.6%, with or without molybdenum up to about 2.5% and the sum of the chromium and molybdenum contents not exceeding about 22.5%, and remainder substantially all iron. Yet another steel essentially consists of about 20% to about 21.5% chromium, about 7% to about 9percent nickel, about 4% to about 6% manganese, with silicon not exceeding about 0.25%, carbon not exceeding 0.020%, nitrogen about 0.25% to about 0.35%, and remainder substantially all iron. A further specific steel, this having strength and excellent resistance to intergranular attack, essentially consists of about 18% to about 22% chromium, about 8% to about 9.5% nickel, about 5% to about 6.5% manganese, silicon not exceeding 0.25%, about 0.2% to about 0.4% nitrogen, carbon not exceeding 0.020%, and remainder substantially all iron.

A steel in which perhaps the best combination of properties is had, notably strength, toughness, general resistance to corrosion, and resistance to intergranular corrosion, essentially consists of about 20.25% to about 21.50 chromium, about 6.5% to about 7.5% nickel, about 5% to less than 7% manganese, silicon not exceeding about 0.25%, about 0.25% to about 0.35% nitrogen, with carbon not exceeding 0.030%, and remainder substantially all iron. This preferred steel more particularly consists essentially of about 20.5% to about 21% chromium, about 7% nickel, about 6% manganese, about 0.25% to about 0.35% nitrogen, carbon not exceeding 0.030%, and remainder substantially all iron.

A preferred specific steel, which additionally includes columbium as an ingredient, essentially consists of about 17.5% toabout 20% chromium, about 6% to about 9.5% nickel, about 4% to about 6.5% manganese, about 0.2% to about 0.35% nitrogen, carbon not exceeding 0.035%, about 0.1% to about 0.6% columbium, and remainder substantially all iron. This steel is strong and tough, as well as having good resistance to intergranular corrosive attack. And a further preferred steel, in which molybdenum is employed as an essential ingredient, a steel enjoying particularly good resistance to chlorides but which is inclined to contain some ferrite, essentially consists of about 17.5% to about 21.5% chromium, about 5% to about 9.5% nickel, about 4% to less than 7% manganese, about 0.2% to about 0.4% nitrogen, carbon not exceeding 0.030%, molybdenum about 1.5% to about 2.5% with the sum of the chromium and molybdenum contents not exceeding about 22.5%, and remainder substantially all iron.

Our steel is melted with known available equipment. It is austenitic in structure. It works well in the hot-mill in conversion from ingot and billet to plate, sheet and strip or into bar and rod stock. Moreover, it readily lends itself to further reduction as by cold-rolling or cold-drawing. The metal is comparatively ductile, readily lending itself to a variety of forming operations such as bending, pressing, drawing, cutting, drilling, threading, or other machining, and even to upsetting. It may be readily welded or brazed employing known and commonly used techniques'And in the as-welded or as-brazed condition enjoys a combination of strength, toughness, corrosion-resistance and resistance to intergranular corrosion, as noted above.

As particularly illustrative of the chromium-nickel-manganese-nitrogen steel of our invention, this as compared with other chromium-nickel-manganese-nitrogen steels, we give below in Table 1 a the chemical composition of a series of such steels, some according to our invention and others of composition departing from the steel of our invention in one or more of the ingredients. in Table 1 b we give indication of the structure of the steels of Table I a and their susceptibility to corrosive attack both in the annealed and in the sensitized condition, both by nitric acid and by ferric sulphate.

TABLE 1a Chemical Composition of Chromium-Nickel- Manganese-Nitrogen Stainless Steels 35 9!: l1: 7; Heat No. C Mn Si Cr Ni N Mo R5855 0.018 2.79 0.32 18.61 6.63 0.34 R5856 0.018 2.90 0.40 18.90 6.63 0.32 2.02 R5858 0.015 2.83 0.37 19.08 9.10 0.31 R5859 0.013 2.74 0.35 19.05 9.21 0.33 2.02 R5971 0.020 2.85 0.24 21.30 6.52 0.37 R 5861" 0.013 2.81 0.42 21.03 6.48 0.33 2.06 R5862 0.015 2.95 0.44 21.32 9.11 0.34 R5863" 0.015 2.83 0.39 21.03 9.20 0.31 2.04

R5893 0.016 6.10 0.49 18.74 6.55 0.33 R5894 0.016 5.86 0.49 18.71 6.65 0.32 2.03 R5891 0.016 6.09 0.40 18.32 9.09 0.33 R5892 0.016 5.95 0.40 18.65 9.22 0.33 2.03 R5895 0.015 6.34 0.40 20.98 6.71 0.34 R5896 0.015 6.09 0.42 20.89 6.70 0.32 2.05 R5897" 0.015 6.18 0.43 21.07 9.14 0.33 R5898 0.015 6.00 0.48 20.88 9.28 0.33 2.05

R65l4-l 0.036 8.74 0.29 20.28 6.64 0.29 R65 14-2 0.030 8.74 0.29 21.08 6.64 0.29 R65 14-3 0.033 8.74 0.29 22.06 6.64 0.29 R65 15-1 0.036 9.40 0.31 20.37 9.06 0.28 R6515-2 0.038 9.40 0.31 21.40 9.06 0.28 R65 1 5-3 0.039 9.40 0.31 22.36 9.06 0.28

R5937 0.026 9.69 0.14 20.49 6.58 0.30 R5938 0.027 9.78 0.13 20.36 6.56 0.34 R5939 0.027 9.89 0.13 20.53 6.62 0.38

Phosphorus 0.004/0.012%. sulphur 0.015/0.013%

Steels according to the invention "Steels according to the invention in which a best combination of properties is had Now the resistance of the steels of Table l a to attack by boiling 65% nitric acid solution (Huey test) where the average rate of attack is given for five periods of treatment, and by ferric sulphate, that is, ferric sulphate-sulphuric acid, in the annealed condition and in the sensitized condition, is given below in Table 1 b. The several steels were in the form of inch diameter inch length cylindrical specimens polished to grit finish. The annealed steels were annealed at a temperature of 1,950 F. for one-half hour and water quenched. The sensitized steels were similarly annealed and then sensitized by heating at 1,250 F. for 1 hour and cooling in air. The figures reported represent the extent of the attack in inches per month (1PM).

The maximum rates of lntergranular attack which are considered to be acceptable and which characterize the steels of our invention are dependent upon the condition of the metal, i.e., annealed or sensitized, and upon the particular corrosive media employed in test, i.e., nitric acid or ferric sulphate. The steels according to our invention (marked by asterisk) meet the requirements that in the nitricacid test the rate of attack does not exceed 0.0010 inches per month (1PM) when in the annealed condition and 0.0020 [PM when in the sensitized condition; and in the ferric sulphate test does not exceed 0.0020 1PM when in the annealed condition and 0.0035 [PM when in the sensitized condition.

In addition to the report on corrosive attack, there is indicated the structure of the steels in the annealed condition as determined by hand magnet.

TABLE 18 Structure of and Rate of lntergranular Attack on the Steels of Table 1a HNO;1PM Fe,(SO,),lPM

Sensi- Sensi- Heat No. Mag. Annealed tizcd Annealed tized R5855 N 0.0010 0.0040 0.0030 0.0263 R5856 N 00008 0.0054 0.0023 0.0031 R5858 N 00007 0.0021 0.0016 0.0077 R5859 N 0.0006 0.0055 0.0015 0.0052 R5971 N 0.0006 0.0012 0.0014 0.0024 "R5861 M 0.0004 0.0005 0.0013 0.0017 R5862 N 0.0004 0.0008 0.0015 0.0025 "R5863 N 0.0004 0.0008 0.0010 0.0015

R5893 N 00009 0.0021 0.0029 0.0084 R5894 VSl 0.0008 0.0024 0.0023 0.0032 R5891 N 0.0007 0.0020 0.0022 0.0038 R5892 0.0006 0.0012 0.0017 0.0021 R5895 VSl 0.0005 0.0006 0.0017 0.0032 R5896 V 00005 0.0179 (3) 0.0015 0.0105 "R5897 N 00005 0.0006 0.0011 0.0016 R5898 S1 0.0004 0.0032 0.0010 0.0015

R6514-2 S1 0.0010 0.0018 0.0053 R65 14-3 M 0.0007 0.0017 0.0040 R6515-l N 0.0212 0.0020 0.0241

R5937 N 0.0010 0.0010 0.0024 0.0023 R5938 N 0.0021 0.0019 0.0026 0.0030 R5939 N 0.0029 0.0049 0.0025 0.0023

116540-2 VSl 0.0014 0.0129

R6540-3 Sl 0.0020 0.0191

(3) Average for 3 periods of test .l. Magnetism Code N nonmagnetic VSl very slightly magnetic S1 slightly magnetic M moderately magnetic V very magnetic Steels according to the invention "Steels according to the invention in which a best combination of properties is had Study of the test data presented above in Table lb, this as it relates to the steels of composition according to Table l a, rather clearly reveals that those having a high-manganese content, although of acceptable chromium, nickel, nitrogen and carbon contents, suffer an excessive loss, particularly when in the sensitized condition. Thus, the steels of Heat Nos. R6514-1, R65l4-2 and R65l4-3, having a manganese content of 8.74% and about 0.033% carbon, and those of Heat Nos. R65l5-1, R6515-2 and R65l5-3, with a manganese content of 9.40% and about 0.038% carbon, are characterized by ferric sulphate attack in the sensitized condition ranging from 0.0040 1PM to 0.0241 IPM. While some of these steels were nonmagnetic, the others were at least slightly magnetic. None was found satisfactory. And the steels of Heat Nos. R5937, R5938 and R5939 of similar chromium, nickel and manganese contents but of even lower carbon contents, carbon being about 0.027%, were not satisfactory because of high-corrosive attack. So, too, the steels of Heat Nos. R6540-2 and R6540-3, of substantially like composition but with intermediate manganese contents respectively of 7.44 and 9.28%, sustained objectionable rates of corrosive attack. The one steel suffered an attack of 0.0129 1PM and the other 0.0191 1PM, both in ferric sulphate and in sensitized condition.

Of the group of steels of Table l a having a chromium content on the order of some 18 to 22%, with nickel ranging from 6.5 to 9.5%, and with manganese on the order of 3%, best results with minimum attack are had with the steels of the higher chromium contents; actually, the steels with thelower chromium contents are not acceptable. Thus, while the had. And, here again, improvement is felt with the presence of molybdenum, providing the sum of the chromium and molybdenum contents does not exceed about 22.5%. Thus, for the 19161653212113; 7 the l 9' m n an se steels, the two steels containing molybdenum (Heat Nos. R5894 and R5892) suffered less attack by ferric sulphate, namely, 0.0032 and 0.0021 1PM, than the two steels free of molybdenum (Heat Nos. R5893 and R5891), these registering attacks of 0.0084 and 0.0038 1PM. But note that it is only the L steelys .2%in c90t t30010920 molybdenum content (Heat No. R5892) which enjoys an acceptable rate of attack by ferric sulphate in the annealed condition and in the sensitized condition as well, namely 0.0017 and 0.0021 1PM respectively.

Somewhat better results are had with the 21-6-6 and the 21-'96 chromium-'nickel-marganese steels. For example both the 21-6-6 (Heat No. R5895) and the 21-9-6 (Heat No. R5 897) are possessed of very low rates of intergranularv attack both in nitric acid and in ferric sulphate and both in the annealed condition and in the sensitized condition. Where, however, molybdenum is present (Heat Nos. R5896 and R5898) and the sum of the chromium and molybdenum contents exceeds about 22.5%, there is a loss in resistance to intergranular attack, particularly in the nitric acid solution. Thus, both the 21-6-6 steel (Heat No.R5896) and the 21-9-6 steel (Heat No. R5898) are seen to suffer excessive attack, the one averaging 0.0179 1PM for only the first three periods, and the other 0.0032 IPM for the usual five. This we attribute to the circumstance that with the high-manganese content and the high chromium and molybdenum contents ferrite appears in the structure, this being of objectionable form as noted below.

The differences in the rate of intergranular attack are in some measure reflected by a difference in mechanical properties. In general, it appears that the high-manganese steel falls somewhat short in both tensile strength and yield strength as compared to the steels of the low and intermediate manganese contents. Thus, there are reported below in Table 1c the mechanical properties of typical steels, the chemical compositions of which are set out in Table la and the rates of intergranular attack given in Table lb. The mechanical properties given are: tensile strength in pounds per square inch, 0.2%

21-9-3 chromium-nickel-maneanese steel (Heat No. R5855 sustained a loss in ferric sulphate in sensitized condition of 0.0263 1PM, the 21-6-3 steel (Heat No. R5971) suffered a loss of only 0.0024 [PM in such condition, both steels having about the same carbon and nitrogen contents, namely about 0.020% and about 0.35% respectively. The other 19-6-3 steel and the 19-9-3 steels (Heat No. R5856 and Heat Nos. R5858 and R5859) note an excessive rate of attack both in nitric acid and in ferric sulphate, particularly in the sensitized condition.

lt is in the low-manganese steels of the higher chromium contents that resistance to intergranular attack is had. Note that the 21-6-3 chromium-nickel-manganese steels (Heat Nos. R5971 and R5861), as well as the 21-9-2 steels (Heat Nos. R5862 and R5863) are characterized by a low rate of attack. Moreover, in these steels the ingredient molybdenum is beneficial, as seen, for example, by comparing the two 21-6-3 chromium-nickel-manganese steels (Heat Nos. R597] and R5861). The molybdenum-free steel R5971 suffered a loss of 0.0024 1PM while the other containing about 2% molybdenum had a loss of only 0.0017 IPM. Like results are had in the 19 -6- 3 chromium-nickel-manganese st'eel where .th'el'rnolyb-' denum-free steel (Heat No. R5862) registered a loss of 0.0025 1PM and the steel containing about 2% molybdenum (Heat No. R5863) had a loss of 0.0015 IPM. In both the 21-6-3 and the 21-9-3 chromium-nickel-manganese steels a molybdenum addition is beneficial, not only for the sensitized steels subjected to ferric sulphate solution, but also to these subjected to boiling nitric acid solution, although the benefit is less pronounced particularly in the 21-9-3 steels.

1n the steels of intermediate manganese content, that is, some 5.5 to 6.5%, as presented in the second grouping in Tables la and lb, a generally lower rate of intergranular attack is Steels according to the invention tElSteels according to the iton in which a best of propertiesis had Of the five steels the mechanical properties of which are set out above, it is the prior art 21-6-9 chromium-nickel-manganese steel (Heat No. R65l4-1) which suffers by comparison. This steel also is the only one of the five which is characterized by an unacceptable rate of intergranular attack, as shown in Table l b. The 21-6-3 chromium-nickel-manganese steel (Heat No. R5971), the 21-9-3 steel (Heat No. RS862), the 21-6-6 steel (Heat No. R5895) and the 21-9-6 steel (Heat No. R5897) all enjoy good strength, as well as good ductility. And as previously noted, all four of these steels enjoy excellent resistance to intergranular attack both by nitric acid and by ferric sulphate, the attack being resisted by the steels even when in the sensitized condition.

Up to this point, little has been said respecting the magnetic properties of our steel as they may relate to the resistance to intergranular attack. Perhaps some further explanation is desirable in view of the circumstance that the molybdenumbearing chromium-nickel-manganese steel Heat No. R5896, which is magnetic, is not satisfactory, as noted above, while the molybdenum-bearing chromium-nickel-manganese steel Heat No. R5861, although magnetic, is most satisfactory.

in general, our steel, being austenitic, is essentially nonmagnetic. Note the specific steels of Tables Ia and lb particularly the 21-6-3 steel (Heat No. R5971), the 21-9-3 steels (Heat No. R5862 without molybdenum and Heat No. R5863 with molybdenum), the 19-9-6 steel with molybdenum (Heat No. R5892) and the 21-9-6 steel without molybdenum (Heat No. R5897). These steels are nonmagneticAnd the 21-6-6 steel (Heat No. R5895) is but very slightly magnetic.

Surprisingly enough, the 21-6-3 chromium-nickel-manganese steel with about 2% molybdenum (Heat No. R5861), one of our best steels in terms of resistance to intergranular attack, is found to be moderately magnetic. The magnetic characteristic we attribute to the presenceof alpha ferrite deriving from high chromium and molybdenum contents as they relate to the comparatively low nickel and manganese contents.

The 21-6-6 steel containing molybdenum (Heat No. R5896), however, is very magnetic as noted above. Moreover, it is seen to be of substantially higher manganese content than the Heat No. R5861. Here we feel that the manganese content, which is over and above about 4 or 5%, serves as a ferrite former rather than an austenite former as with the lower manganese contents. And as a result that there is had a substantially higher ferrite content. With the sensitizing treatment this ferrite is partially converted to the chi phase, which phase renders the steel highly susceptible to corrosive attack. But irrespective of theoretical considerations, considerations by which we prefer not to be bound, the plain fact is that the chromium-nickel-manganese steel containing molybdenum where manganese is on the high side, as in the Heat No. R5896, not only is magnetic but is also susceptible to intergranular attack, and a similar steel but with manganese on the low side is virtually free of attack.

A best combination of strength, ductility, general corrosionresistance, freedom from intergranular attack, and freedom from magnetic effects is had in our steel where the manganese content, as noted above, amounts to about 5%, or more broadly, some 3 or 4% to some 6 or 6.5%, this with chromium on the order of about 20% to about 22.5% and nickel about 6% to about 9.5%, with carbon not exceeding 0.040% and nitrogen about 0.25% to about 0.35%. Actually, a best combination of strength and resistance to intergranular attack is had with the steels having a manganese content not exceeding about 6%.

Thus, in conclusion, it will be seen that we provide in our invention a stainless steel in which there are achieved the various objects set out above, and in which there are realized the various advantages noted. The steel of our invention enjoys a combination of strength, toughness and resistance to corrosion. It is particularly suited to a wide variety of applications in which the metal may be fabricated by welding, brazing, or the like. The steel and various products fashioned thereof may be put to use in the as-welded condition, that is, without necessity for annealing or other heat treatment following the welding operation.

inasmuch as numerous embodiments may be made of our invention and numerous changes made in the embodiments shown, notably many variations made in the compositions of the specific steels hereinbefore set forth, it is to be understood that all matter described in our specification is to be interpreted as illustrative and not by way of limitation.

We claim: 31. Alloy steel essentially consisting of about 17.5% to about 21.5% chromium, about 5% to about 9.5% nickel, about 4% to less than 7% manganese, carbon not exceeding 0.030%, nitrogen about 0.2% to about 0.4%, about 1.5% to about 2.5% molybdenum with the sum of the chromium and molybdenum contents not exceeding about 22.5%, and remainder substantially all iron. I

2. Alloy steel essentially consisting of about 20% to about 22.5% chromium, about 5% to about 7.5% nickel, about 2.5% to about 5% manganese, carbon not exceeding 0.030%, nitrogen about 0.25% to about 0.4%, and remainder substantially all iron.

3. Alloy steel essentially consisting of about 20% to about 22% chromium, about 7.5% to about 9.5% nickel, about 2.5% to about 4% manganese, silicon up to about 0.5%, carbon not exceeding 0.030%, nitrogen about 0.25% to about 0.35%, and remainder substantially all iron.

4. Alloy steel essentially consisting of about 20.25% to about 21.50% chromium, about 6.5% to about 7.5% nickel, about 5% manganese, silicon not exceeding about 0.25%, carbon not exceeding 0.030%, nitrogen about 0.25% to about 0.35%, and remainder substantially all iron. 

2. Alloy steel essentially consisting of about 20% to about 22.5% chromium, about 5% to about 7.5% nickel, about 2.5% to about 5% manganese, carbon not exceeding 0.030%, nitrogen about 0.25% to about 0.4%, and remainder substantially all iron.
 3. Alloy steel essentially consisting of about 20% to about 22% chromium, about 7.5% to about 9.5% nickel, about 2.5% to about 4% manganese, silicon up to about 0.5%, carbon not exceeding 0.030%, nitrogen about 0.25% to about 0.35%, and remainder substantially all iron.
 4. Alloy steel essentially consisting of about 20.25% to about 21.50% chromium, about 6.5% to about 7.5% nickel, about 5% manganese, silicon not exceeding about 0.25%, carbon not exceeding 0.030%, nitrogen about 0.25% to about 0.35%, and remainder substantially all iron. 